1. Field of the Invention
The present invention relates to a laser light irradiation method and a laser treatment apparatus for laser light irradiation. The invention also relates to a method for manufacturing a semiconductor device using laser light irradiation.
2. Description of the Related Art
A laser treatment apparatus which performs various kinds of processing by laser light irradiation and a laser light irradiation method have been known.
One feature of using laser light is that only a region which is irradiated with laser light and absorbs energy thereof can be selectively heated unlike in the case of heat treatment using radiation heating or conduction heating. For example, laser treatment using an excimer laser oscillator, which oscillates ultraviolet light having a wavelength of 400 nm or less, heats a semiconductor film selectively and locally, and achieves crystallization or activation of the semiconductor film with little thermal damage to a glass substrate.
A method for laser-annealing a semiconductor film formed over a large-size glass substrate with a linear laser is explained as an example of the laser light treatment apparatus and the laser light irradiation method with reference to FIGS. 2A to 2C. First, a semiconductor film 1002 formed over a substrate 1001 is scanned by a linear laser 1003, whose beam spot 1004 has a longitudinal length L of several hundred micrometers, in a width direction of the linear laser (in the direction of an arrow in FIG. 2A). Then, a region of several hundred micrometers in the semiconductor film 1002 is laser-annealed (FIG. 2A). Subsequently, the linear laser 1003 is translated (is moved) by as much as the longitudinal length L of the linear laser 1003 (FIG. 2B), and the semiconductor film 1002 is scanned again by the linear laser 103 in a width direction (FIG. 2C). By repeating this process several hundred to thousand times, the semiconductor film 1002 formed over the large-size substrate 1001 can be entirely laser-annealed.
FIGS. 2A to 2C show the case of using a linear laser. In the case of shaping a continuous wave laser beam converted into a harmonic into a linear laser, a longitudinal length of the linear laser needs to be as extremely short as several hundred micrometers to obtain a power density sufficient to laser-anneal a semiconductor film. Therefore, in the case of entirely laser-annealing a semiconductor film formed over a large-size glass substrate, for example, a glass substrate having a side of approximately one meter, the substrate needs to be placed on a movable table such as an XY stage which can be moved right to left and up and down, and the movable table provided with the substrate needs to be reciprocated several hundred to thousand times.
Note that, in this specification, a direction of a longer axis of a linear laser is referred to as a long-axis direction or a longitudinal direction, and a direction of a shorter axis of a linear laser is referred to as a short-axis direction or a width direction.
Meanwhile, an injury, a fire, and the like are major disasters which occur in performing laser annealing. When a combustible material is irradiated with laser light, the combustible material has possibility of igniting by absorbing the laser light and generating heat. Therefore, especially against a fire, it is very important to take preventive measures in advance (Reference 1: Japanese Patent Laid-Open No. 9-174264, Reference 2: Japanese Patent Laid-Open No. 10-263866, Reference 3: Japanese Patent Laid-Open No. 2001-18079, and Reference 4: Japanese Patent Laid-Open No. 2003-126977).
It is an object of the present invention to safely perform laser annealing without irradiating an unintended region with laser light so as not to cause a disaster even when an unusual situation is brought about in performing laser annealing.
A disaster such as a fire in performing laser annealing is thought to be caused by an abnormal stop of a stage which moves a substrate (for example, an XY stage), vibration of an apparatus due to an earthquake or the like, generation of an abnormal gas, generation of smoke, a change in an optical system such as a mirror due to high heat, or the like.
When a stage is abnormally stopped at an unexpected position during laser annealing, a portion below an intended irradiation position may ignite, or a portion therearound may ignite due to reflected light from an unspecified position or the like, which is very dangerous. As a cause of a stage stop at an unexpected position, freeze (abnormal stop) of a system control computer (for example, a personal computer (PC)), abnormality in communication between a PLC and a PC, a malfunction of an electronic device due to noise from outside, or the like can be given.
When an optical element such as a mirror falls or a reflection angle or an incident angle is changed due to apparatus vibration caused by an earthquake or the like, a portion different from a portion normally irradiated with laser light is irradiated with the laser light. Therefore, the portion may ignite.